Drilling fluid (“drilling mud”) is a fluid used during the process of drilling boreholes into the subsurface strata for hydrocarbon extraction. Generally, the drilling fluid lubricates and cools the drill bit during drilling operations, seals off the wall of the bore, effectively isolating the bore from the formations, lifts the drilling cuttings from the bit to surface, and provides the primary defense for well control. Drilling fluids are typically classified into three categories: (1) water-based; (2) non-water based; and (3) pneumatic. The discussion below is based on non-water based fluid systems.
Generally, the drilling fluid and required additives are supplied to the drilling rigs by various vendors and depending on the type of equipment used, and the types of formations to be encountered, the formula and properties of the drilling fluid chosen will vary.
Typically, when recycling oil-based mud, it becomes necessary to build the volume of the system in order to dilute the weight of the drilling fluid, adjust the oil/water ratio, or simply to achieve the required volume for the drilling operation. In the standard process, diesel fuel is added into the system until the volume is such as to satisfy the oil-water ratio of the planned final total volume of drilling fluid. Following that, the emulsification additives are added in designated proportions, based on manufacturer's specification or other analytical data, and allowed to mix under the proper shear and temperature conditions to allow for a stable emulsion between the oil and water phases to be created.
At this point, other additives can be added based on the specific rheology requirements or anticipated reactions that will occur with the geologic formations being contacted. For example, lime is typically added to aid in the reaction yielding emulsification. Finally, brine, typically consisting of 25%-38% by weight of calcium chloride (CaCl2), is slowly added into the slurry until the volume of brine added satisfies the desired oil/water ratio of the final design.
During the input of the brine and all other additives, the drilling fluid continues to mix under the proper shear and temperature conditions to allow for a stable emulsion to be created. The quality of the emulsion is then measured by a probe reporting a unit of measure known as the electric stability, which is a relative measure of the voltage required to break down the emulsion. An electric stability reading greater than 500 units is typically considered by industry standards to be representative of a quality stable emulsified system.